1. Field of the Invention
The present invention relates to drive circuits for driving motors, and particularly, to drive circuits employing sine-wave PWM for driving motors such as brushless motors and induction motors.
2. Description of the Related Art
FIG. 1 shows a drive circuit employing sine-wave PWM (pulse width modulation) for driving a motor, according to a prior art. The drive circuit 10 has a standard clock generator 1 for generating a standard clock signal. In synchronization with the standard clock signal, a timing circuit 2 specifies a ROM address, reads data from a ROM 3 according to the specified address, and transfers the read data to data latches 4. The ROM 3 stores data and corresponding addresses. The latches 4 temporarily store data read out of the ROM 3. DACs (digital analog converters) 5 are connected to the latches 4, respectively. A triangular wave generator 6 generates a triangular wave serving as a carrier signal for PWM. PWM generation comparators 7 are connected to the DACs 5, respectively. The latches 4, DACs 5, and comparators 7 are arranged for U-, V-, and W-phases, respectively.
Based on the standard's clock signal from the clock generator 1, the timing circuit 2 specifies a ROM address, reads data (voltage value) from the ROM 3 according to the specified ROM address, and transfers the read data to the latches 4 at the U-, V-, and W-phase timing. The DACs 5 receive the data from the latches 4 and convert the data into analog voltages to form basic waves used to generate sine-wave currents. The basic waves and a triangular wave from the triangular wave generator 6 are used by the comparators 7 to form, through PWM operations, sine-wave currents for driving a motor (not shown).
To prepare the basic waves for generating the sine-wave currents, the prior art of FIG. 1 must read data from the ROM 3 and convert the read data into voltages (the basic waves) through the DACs 5. This scheme increases the size of the drive circuit 10, and when the drive circuit 10 is integrated into a specific IC, raises the cost thereof.
To correctly generate the sine-wave currents, the drive circuit 10 must have precision, high-resolution DACs, which increase the size and cost of the drive circuit 10.